The present invention relates to a high-efficiency speech encoding/decoding apparatus for use in a pulse code-modulation (PCM) comnunications system, and more particularly to a burst error correction circuit for speech encoding/decoding apparatus using a speech-adaptive predictive coding system.
For the prevention of speech quality deterioration owing to errors in information bit sequences in this sort of high-efficiency speech encoding/decoding apparatus, typically the following two methods are known.
The first uses error correction codes. This method, though effective against random errors and short burst errors, has to use codes of low efficiency (the number of information bits/block-length) when used on a transmission path where great fluctuations occur over time and random errors and long burst errors also arise, such as in mobile communication. This results in a greater number of redundant bits with a diminishing effect on the advantage of using a high-efficiency speech coding system.
The second relies on waveform interpolation. Utilizing the quasi-periodic nature of the waveforms of speech (especially voice), this method interpolates the waveform prior by one pitch period (hereinafter referred to as waveform interpolation) when detecting a burst error, which can be detected with comparative ease (See FIGS. 1(A) and 1(B).) This second method is used for analog speech transmission or the transmission of speech waveforms alone, such as in a PCM communications system. However, the method has to be improved if it is to be applied to an encoding apparatus, such as an adaptive predictive encoding system in which parameter information also has to be transmitted. It also requires the detection of the pitch period and accordingly a circuit of greater dimensions.
Another method has been contemplated by the inventor herein to improve the second technique. It utilizes the time-correlation of speech (the relative slowness of the fluctuations of formant and sound intensity) to substitute the prior bits for parameter information as well. This method will be hereinafter referred to as word-by-word parameter interpolation.
This interpolation method replaces all the bits constituting a parameter with the bits of the prior frame even if only a few of the bits are erroneous. Therefore, the frequency of parameter replacements (hereinafter referred to as the parameter interpolation rate or simply the interpolation rate) will become too great if each parameter consists of a large number of bits. Especially where this technique is used in combination with the first method to achieve transmission of interleaved signals after the coding of error correction, burst errors will be randomized. If the errors are within the error correcting capacity of the apparatus, there will be no problem. But, if errors beyond the capacity are detected by the burst error detecting circuit, there will arise such an awkward situation that, for example, every parameter includes one error bit and therefore must interpolate or replace all the parameters with all the parameters of the preceding frame.
In the conventional system, either the natural binary code or the folded binary code has usually been employed for the binary encoding of the quantized level of parameters. In these coding systems, replacement of only the error bits, or the bits highly likely to be erroneous, with the bits of the respectively preceding frames cannot suffficiently correct these error bits and therefore, deteriorates the quality of the decoded speech signal, which will be described in detail later.